1. Field of the Invention
The present invention relates to a traffic information display device for displaying traffic information delivered thereto on a map image displayed on the display screen thereof, a method of displaying traffic information delivered thereto, and a recording medium on which a display control program for use in the traffic information display device is recorded.
2. Description of the Prior Art
In general, when a vehicle is moving in a well-trafficked area or an area including a number of roads, a vehicle navigation device carried in the vehicle receives a large amount of traffic information. Therefore, when all the traffic information delivered to the vehicle navigation device is displayed on the screen thereof, the amount of on-screen information is too large for the user to easily recognize the traffic information displayed on the screen of vehicle navigation device. Particularly, since most roads in a well-trafficked area such as a midtown area are congested or dense, a large amount of traffic jam information displayed on the traffic information display device makes the visibility of the display screen worse. When a number of display marks such as red lines each showing a congested area of each road are displayed on the screen, they make it more difficult for the user to easily recognize all the on-screen information.
A vehicle navigation device as disclosed in Japanese Patent Application Laying Open (KOKAI) No. 8-63094 intended to solve the above problem is known. Referring now to FIG. 26, there is illustrated a block diagram showing the structure of the prior art vehicle navigation device. In FIG. 26, reference numeral 1 denotes a bearing sensor for measuring the direction in which the vehicle is headed, 2 denotes a speed sensor for measuring the speed of the vehicle to furnish a signal with pulses the number of which is determined by the vehicle speed, and 3 denotes a map storage memory for storing road map data including intersection network data.
Furthermore, reference numeral 4 denotes a central processing unit (CPU) which receives information about the vehicle from the bearing sensor 1, speed sensor 2, and so on, and then performs various types of operations on the information about the vehicle and display control processing, 5 denotes a ROM for storing a control program and the like which are executed by the CPU 4, and 6 denotes a RAM for storing operation results from the CPU 4.
In addition, reference numeral 7 denotes a VRAM for storing image data generated by the CPU 4, 8 denotes a display such as a cathode-ray tube display or a liquid crystal display, on the screen of which display data such as map information and traffic jam information are displayed, 9 denotes a console panel through which the user can select information to be displayed on the screen of the display 8, 10 denotes a global positioning system (GPS) receiver which receives radio waves from GPS satellites to detect the position of the vehicle, and 11 denotes a traffic information receiver which receives various types of road traffic information such as traffic jam information transmitted thereto through a beacon or an FM multiplex broadcasting.
Furthermore, reference numeral 12 denotes an interface circuit which can transmit a signal among the bearing sensor 1, speed sensor 2, map storage memory 3, CPU 4, ROM 5, RAM 6, VRAM 7, display 8, console panel 9, GPS receiver 10, and traffic information receiver 11.
In operation, the CPU 4 detects the current position of the vehicle by means of the GPS receiver 10 first, and then reads road map data about an area in the vicinity of the current position from the map storage memory 3. When the vehicle navigation device receives traffic information, the CPU 4 computes the mean value of the extent of traffic congestion in all links to be displayed on the display 8, and after that computes the number of the links to be displayed on the display 8.
When the number of the links is equal to or larger than a threshold value A and the mean value of the extent of traffic congestion is larger than a threshold value B, the vehicle navigation device displays all congested links on the screen of the display 8 with a thick line having a predetermined color. On the other hand, when the number of the links is equal to or larger than the threshold value A and the mean value of the extent of traffic congestion is smaller than the threshold value B, the vehicle navigation device displays all links not congested on the screen of the display 8 with the thick line having the predetermined color.
Thus, the prior art traffic information display device is adapted to reduce the difficulty in recognizing traffic information on the display screen by emphasizing the display of the on-screen congested links when it determines that the mean extent of traffic congestion exceeds the predetermined threshold value.
While the prior art traffic information display device can emphasize the display of all congested roads on the screen thereof or vary the number of roads displayed on the screen thereof to reduce the difficulty in recognizing the whole of the display screen, the prior art traffic information display device suffers from the disadvantage that the user cannot recognize needed information because it does not make a distinction between information necessary for the user and information unnecessary for the user.
For example, since the traffic information display device emphasizes the display of all congested on-screen roads when it receives a large amount of traffic jam information, there may be a case where the user cannot recognize needed traffic jam information about an area in the vicinity of the vehicle and the vicinity of a path through which the vehicle is scheduled to move. On the other hand, the variation of the number of on-screen roads may cause the disappearance of needed road information from the display screen.